Currently, ultrafiltration (UF), microfiltration (MF) separation membranes have been widely used in food industry, medical fields, domestic and municipal water usage, and industrial and municipal waste water treatments and the like. Presently, there is a severe shortage of water resources around the world. Great promotion of water savings and waste water discharge reduction, treatment of industrial waste water and municipal waste water, and further in-depth treatment are to achieve the objective of re-use of neutral water, so as to realize the overall goal of “zero emission”.
In recent years, the filtration method of porous hollow fiber membranes has gradually been used widely, mainly in treatment of industrial and municipal waste water and further treatment so as to achieve the re-use of neutral water. This method treats water in large quantity, and has the safe characteristics of water treatment, large effective membrane surface areas per unit volume, and small device space. This method requires that the porous hollow fiber membranes used have high porosity and narrow pore size distribution to improve separation efficiency and separation accuracy. Moreover, it also requires that the membranes possess the pore sizes most suitable for separation targets, and the characteristics of effectively excluding bacteria, suspension solids, and turbid components. Meanwhile, the membrane fibers of the membranes shall have higher mechanical strength and high water flux so that they can be long-term used under the conditions of chemically cleaning of polluted membranes and operations under high operational pressures.
China patents (CN9511749T and CN98103153) disclose an earlier method for producing polyvinylidene fluoride (PVDF) hollow fiber membrane by non-solvent induced phase separation (NIPS) technique. Thermoplastic macromolecule polymer resins, good organic solvents and pore forming agents in a ratio are mixed; after homogeneously dissolved, being extruded through a spinning nozzle into the coagulation bath comprising solvents; the good solvents and pore forming agents in the polymer solution are extracted into the coagulated phase, and the polymers are due to phase transfer so as to precipitate and form polymeric hollow fiber membranes. However, in this kind of solvent phase transfer, it is difficult to cause evenly phase separation along the membrane thickness direction, resulting in the formation of an asymmetric membrane that contains a dense surface layer and a supporting layer with finger-like and sponge-like macrovoids. Hence, the membrane has poor mechanical strength owing to isotropic and non-oriented molecules. Furthermore, the exchange of solvents in the process causes a portion of solvents to participate in polymeric gelation, resulting in lower porosity.
In order to solve the problem of the poor mechanical strength of the hollow fiber membranes made by the non-solvent induced phase separation (NIPS) technique, the woven braid made of macromolecule polymers with excellently high mechanic strength or tubular braid is used as the supporting structure for separation membranes, and the embedding technique or coating technique is used to produce hollow fiber membrane or tubular membrane with strengthened supporting structure. Hayano et al. (U.S. Pat. No. 4,061,821) disclose a membrane with a completely embedded tubular braid; the membrane has a larger thickness so as to increase fluid flowing resistance and then remarkably reduce water permeability. U.S. Pat. No. 5,472,607 describes the use of thin films and current flat sheet membrane coating techniques to produce a composite hollow fiber membrane having a layer of functional separation thin film being coated onto the surfaces of the reinforced material or supporting material made of tubular braids. Even though it solves the problem of the increased fluid flowing resistance in the embedding approach, no matter of the embedding approach or coating technique causes the separation of separation membrane and supporting material during usage because membrane-producing material especially thermoplastic macromolecule polymer resins such as polyvinylidene fluoride (PVDF) has different physical properties from other macromolecule polymer braid materials, resulting in weak binding force between them. For example, under higher temperature, polyacrylnitrile supporting material will contract and cause peeling. The physical properties of woven or knitted braids of polyethylene (PE), or Polyethylene terephthalate (PET), or nylon are different from polyvinylidene fluoride (PVDF), making peeling more easily when being long-term used under pH>10 condition. In addition, the non-solvent induced phase separation (NIPS) technique depends on many parameters, affecting membrane structures and properties. Thus, the operation procedure of membrane production is hard to control, and lacks of reproducibility.
To overcome above-mentioned shortcomings, other membrane production methods have been explored. It was tried to use the thermal induced phase separation (TIPS) technique using heat to induce phase separation. For example, polyvinylidene fluoride with excellent crystallization property simultaneously crystallizes and forms membrane during phase separation in the phase separation process. Australia patent No 653528 discloses an earlier method for making a hollow fiber membrane using the thermal induced phase separation (TIPS) technique. In the disclosed method, polyvinylidene fluoride resin and organic pore-forming agents are mixed and heated under partial vacuum at 220° C. to form a melted polymeric dope; then the polymeric dope is extruded through a spinning nozzle to form a hollow fiber membrane at 215° C.; such a hollow fiber membrane has low porosity. U.S. Pat. No. 5,022,990 discloses another improved method: adding inorganic pore-forming particles into organic pore-forming agents, mixing with polyvinylidene fluoride resin, then melting the mixture and extruding to obtain a hollow fiber-like, or tube-like, or flat-like membrane, and finally extracting to remove the organic pore-forming agents and inorganic pore-forming particles; the resultant membrane has an inner diameter of 1.10 mm, wall thickness of 0.45 mm, mean pore size of 0.05-5 μm, and tensile strength of 7-20 MPa. The shortcoming of this technique is that as the inner diameter of the membrane increases, the wall becomes thinner, reducing pressure resistance and water flux. In order to improve the shortcomings present in membrane production techniques, Japan patent JP3-215535 discloses a method for producing a polyvinylidene fluoride (PVDF) hollow fiber membrane with higher mechanical strength and pressure resistance. polyvinylidene fluoride resin is mixed with organic pore-forming agents such as dioctyl phthalate (DOP) and inorganic pore-forming particle like hydrophobic silicon dioxide nano particles. At the temperature of 250° C., melting and extruding to form a solidified produce, and extracting the organic and inorganic pore-forming agents/particles to obtain a homogeneous pore size symmetric membrane without skins on the surfaces of inner and outer layers; such a membrane has lower filtration accuracy and porosity. Since the outer layer of the membrane produced by the thermal induced phase separation (TIPS) technique is rougher than that of membrane by the non-solvent induced phase separation (NIPS) technique, it tends to reduce the membrane resistance to contamination.